In a scanning tunneling microscope ("STM") an electrically conductive microscope tip is moved in a scanning fashion over the surface of a sample. A pointed end of the microscope tip moves in close proximity to the surface, typically within the space of the diameters of several atoms (approximately within 0.5 nm). Because of the close proximity of the tip to the surface, the probability density function of electrons for atoms in the tip overlaps in space the probability density function of electrons for atoms on the surface. The surface of the sample over which the conductive microscope tip moves is also electrically conductive. Under these conditions, a tunneling current can flow between the microscope tip and the surface, if a suitable bias voltage between these two conductor's is applied. Typically, for a tip-to-sample spacing of a few nm, 100 millivolts of bias voltage will provide on the order of 1 nanoampere of current.
Scanning tunneling microscopes were first built by Binnig and Rohrer (IBM J. RES. DEVELOP., VOL. 30, NO. 4, JULY 1986, pp. 355-369, G. Binnig and H. Rohrer, "SCANNING TUNNELING MICROSCOPY", and IBM TECHNICAL DISCLOSURE BULLETIN, VOL. 27, NO. 10B, MARCH 1985, pp. 5976-5977, G. Binnig et al. "FAST SCAN PIEZO DRIVE"). The scanning tunneling microscope described in the cited publications used a piezoelectric tripod to support and move the microscope tip. This tripod consisted of three rods of piezoelectric material joined at a junction; each rod piezoelectrically expanded and contracted along one of three Cartesian coordinate axes. The microscope tip was mounted at the junction of the three rods. The tip was brought into proximity of the surface by a rough positioner. Thereafter, the piezoelectric rods of the tripod were used to scan the microscope tip across the surface to develop an image of that surface.
U.S. Pat. No. 4,912,822 granted Apr. 3, 1990 based on application Ser. No. 348,707 which is a division of application Ser. No. 149,236 ("the '822 patent")--disclosed a method of making an integrated scanning tunneling microscope. The '822 patent described an integrated tunneling microscope and an integrated piezoelectric transducer and methods for making both. The integrated tunneling microscope included one- or two-arm piezoelectric bimorph cantilevers formed by micromachining using standard integrated circuit processing steps. These cantilevers were attached to the substrate at one area and were free to move under the influence of piezoelectric forces. The piezoelectric forces were caused by the application of appropriate control voltages generated by control circuitry and applied to pairs of electrodes formed as an integral part of the bimorph cantilever structure. The electric fields caused by the control voltages caused the piezoelectric bimorphs to move in any desired fashion within ranges determined by the design. The bimorph cantilevers had tips with sharp points formed thereon by evaporation deposition of a conductive material through a shadow mask. The tips were moved by the action of the control circuitry and the piezoelectric bimorphs so as to stay within a small distance of a conducting surface. However, conventional scanning tunneling microscopes controlled by piezoelectric materials generally have a disadvantage of exhibiting a hysteresis effect which is not ideal for a high absolute accuracy.
European Patent Specification No. 0 194 323, published Aug. 2, 1989 based on European Patent Application No. 85102554.4 filed Mar. 7, 1985, ("the 323 published application") disclosed an integrated form of a scanning tunneling microscope where all movements in the X, Y and Z direction were under the control of electrostatic forces. The '323 published application described a scanning tunneling microscope integrated on a semiconductor chip into which slots were etched to form a center portion linked by a first pair of strips to an intermediate portion. The intermediate portion in turn was linked by a second pair of strips to the main body of the chip. The slots were etched to have mutually orthogonal directions to allow the center portion to perform movements in the X and Y direction under the control of electrostatic forces created between the strips defined by the slots and their opposite walls. A protruding tip was formed on the center portion which was capable of being moved in the Z direction by meal-is of electrostatic forces. The integrated scanning tunneling microscope of the '323 published application was fabricated in such a way that the X, Y and Z movements were not decoupled from each other. Such coupling of the X, Y and Z movements can complicate the control of the tip movement and can detract from the accuracy of tip positioning. Moreover, it would appear to be difficult to fabricate successfully the integrated scanning tunneling microscope described in the '323 published application from a single piece of material.
Thus, a need has arisen for a scanning tunneling microscope head which can be easily fabricated using standard semiconductor integrated circuit fabrication processes and which permits accurate positioning of the microscope tip.